For crossflow microfiltration and ultrafiltration, it is generally recognized that maintaining a uniform transmembrane pressure along the length of a membrane module is advantageous. For such membrane modules a feedstock is circulated from a feed end of the module to a retentate end of the module. A relatively high crossflow velocity is desired to minimize concentration polarization of feed constituents retained at the membrane surface so as to be able to maintain a high and relatively stable membrane flux over time. This high crossflow velocity also results in a pressure drop from the feed end of the module to the retentate end. This pressure drop can be as high as 2 to over 4 bar for membrane modules, depending on the crossflow velocity, fluid viscosity, and membrane feed channel hydraulic diameter and length. For microfiltration, in particular, it can be desirable to operate at a transmembrane pressure (TMP) of less than 1 bar. This is desirable first, for maintaining a relatively stable flux by minimizing concentration polarization and accompanying membrane fouling. Second, operation at low TMP can minimize membrane pore plugging. Third, for some applications, the polarization layer (intensified at high TMP) can become a dynamic membrane and retain feed species that are desired to be removed in a permeate stream. For a feed to retentate pressure decrease from, for example, 5 bar to 2 bar along a membrane module, and a permeate pressure level of 1 bar, the TMP would decrease from 4 bar at the module inlet to 1 bar at the retentate outlet. It is preferred to operate a membrane module at a constant TMP, e.g., 1 bar, and this is the object of the present invention for specific membrane configurations.
One means of equalizing the TMP from a feed end of a membrane module to the retentate end is to circulate permeate in co-current flow along the exterior of a membrane element or elements contained in a permeate collection housing. The pressure drop for the flowing permeate can be controlled to match the feed to retentate pressure drop, thereby resulting in a controllable and essentially constant TMP along the length of the membrane elements. This means was disclosed in 1978 by Sandblom, in U.S. Pat. No. 4,105,547, assigned to Alfa-Laval AB. A subsequent patent also assigned to Alfa-Laval (Holm, et al. in U.S. Pat. No. 4,906,362) further included the addition of a granular filler to the permeate space around a multiplicity of membrane elements in a housing. This filler served to create an increased resistance in the permeate cavity for permeate flow. This reduced the required permeate flow rate to achieve nearly constant TMP in the module, simplifying a system by reducing permeate circulation flow and resulting equipment size and cost, and also reducing power consumption.
An alternative to filling the permeate cavity in a multi-element membrane module to reduce the required permeate circulation flow has been patented by Osterland, et al. in U.S. Pat. No. 5,906,739. In this patent, means are disclosed to encase individual membrane elements within individual permeate collection tubes, creating a small annular space between the membrane element and the permeate tube. The permeate flow through such annuli required to create the desired pressure drop in the co-current permeate flow is reduced from that that would be present in a membrane device with a multiplicity of membrane elements contained only in a housing without the flow restriction disclosed by Holm.
In general, the above techniques are applicable to membrane elements in both tubular and multi-channel configuration. However, the latter are preferred for cost reasons. For these multi-channel membrane devices, two other means for control of TMP along the length of the elements have been commercialized. First is the grading of the permeability of the membrane support along its length, as disclosed by Garcera and Toujas in U.S. Pat. No. 6,375,014. In this technique, the resistance of the membrane support is greater at the feed inlet than it is at the retentate outlet. Within a range of membrane flux, the flow resistance of the support provides a higher pressure drop for permeate flow to the permeate collection zone at the inlet of the element than at the outlet. A second technique has been commercialized by TAMI Industries, and this consists of varying the membrane thickness and resistance along the length of a membrane element. A thicker membrane at the inlet provides a higher pressure drop for permeate flow at the element inlet than at the element outlet, thereby providing a more uniform TMP along the length of the membrane element (Grangeon, et al., U.S. Pat. No. 6,499,606, USP 2003/0070981 A1).
The above methods have been developed primarily for membrane modules that contain multiple elements within a single housing, with the elements principally being of small-diameter, multi-channel construction. A different membrane module structure has been developed by Goldsmith, et al., disclosed in U.S. Pat. Nos. 4,781,831, 5,009,781, 5,108,601, and 6,126,833, all incorporated herein by reference. In these patents, large-diameter, multi-channel membrane elements are disclosed. The distinguishing characteristic of these elements is that they contain one or more internal permeate conduits for extracting permeate from the interior of the elements, circumventing a permeate-side pressure drop within multi-channel monolith membrane devices that would otherwise limit performance of such devices.
A preferred embodiment of the Goldsmith devices includes permeate extraction channels formed at one or both ends of a large diameter monolith, preferably in the form of slots intersecting permeate chambers extending longitudinally along the length of the membrane element. The presence of these channels, when present at both ends of the membrane element, affords a novel means of circulating permeate flow through the permeate conduit, co-currently with feed flow, to achieve a more uniform TMP along the length of the membrane element.
Another embodiment of the Goldsmith devices includes a means of permeate extraction from a monolith membrane element using permeate ducts situated at the end faces of the membrane element, the ducts communicating with the internal permeate conduit chambers through internal transverse channels. The presence of these ducts, when present at both ends of the membrane element, affords a second novel means of circulating permeate flow through the permeate conduit to achieve a uniform TMP along the length of the membrane element.
These constructions are amenable to co-current permeate circulation flow to maintain a constant or near-constant TMP along the length of such monolith membrane elements and form the basis for the present invention.